Representation of Audio Signals 495to arranging the encoded data in the required format. The principle in this encoder is
similar to that planned for the Philips digital compact cassette system.
The exploitation of the masking thresholds in human hearing lies behind many of the
proposed methods of achieving bit rate reduction. One signifi cant difference between
them and conventional PCM converters is the delay between applying an analogue
signal and the delivery of the digital sample sequence. A similar delay is involved when
the digital signal is reconstituted. The minimum delay for a MUSICAM encoder is in
the region of 9 to 24 ms depending on how it is used. These delays do not matter for a
program that is being replayed but they are of concern when the coders are being used to
provide live linking program material in a broadcast application.
A second, potentially more damaging, problem with these perceptual encoders is that there
has been insuffi cient work carried out on the way in which concatenations of coders will
affect the quality of the sound passing through. Although this problem affects the broadcaster
more, the domestic user of such signals may also be affected. Be sure that perceptual coding
techniques remove data from the original, as these data cannot be restored. Thus a listener
who wishes to maintain the highest quality of audio reproduction may fi nd that the use of his
preamplifi er’s control or room equalizer provides suffi cient change to an encoded signal that
the original assumptions concerning masking powers of the audio signal may no longer be
valid. Thus the reconstituted analogue signal may well be accompanied by unwelcome noise.
References .............................................................................................................
There are no numbered references in the text but the reader in search of more detailed
reading may fi rst begin with some of the texts listed here. One risk exists in this
multidisciplinary fi eld of engineering and that is the rate at which the state-of-the-art
of digital audio is being pushed forward. Sometimes it is simply the process of ideas
that were developed for one application area (e.g., submarine holography) becoming
applicable to high-quality audio processing.
A useful overall text is that of John Watkinson ( The Art of Digital Audio , Butterworth-
Heinemann, ISBN 0-240-51270-7).
No text covers every aspect of the subject and a more general approach to many topics
can be found in the oft-quoted Rabiner and Gold ( Theory and Application of Digital
Signal Processing , Rabiner and Gold, ISBN 0-13-914-101-4). Although it was initially